The plugin directory’s licensing guidelines have been updated. The guidelines will now allow code that is licensed under (or compatible with) version 3 of the GPL.

The guidelines still encourage use of “GPLv2 or later,” the same license as WordPress. However, we understand that many open source libraries use other licenses that are nonetheless compatible, such as GPLv2 only, GPLv3, and Apache 2.0.

Now may be a good time for plugin authors to review their plugins to ensure a license is specified. You can add License and License URI headers to readme.txt and the plugin’s headers. (You may also wish to include a copying permission statement.) For example:

This change brings the guidelines in line with the themes directory, which has for some time accepted GPLv3-compatible code. (Probably a good time to note that Creative Commons licenses are still incompatible with the GPL, and the theme and plugin directories.)

It would be nice to have a list with (in)compatible licenses for users which aren’t familiar with this topic.
Since it’s also a problem if your plugin is GPL but your are using an external lib which is incompatible and you didn’t know that.

It’s good practice to include a license.txt or COPYING file. At the very least, you should probably include the copying permission statement, which would state, “You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.” At worst, as long as it is specified somewhere in the readme or code, at least people know what your intent is.

And, if it is in the readme, we will be able to show it on your plugin’s page in the future.

I used the readme validator twice today. it worked earlier this afternoon then failed this evening. I was expecting some explanation would surface eventually. Currently the validator has trouble with the License: lines – sometimes suggesting that the description is too long as well

thanks, now I have more work ensuring all my source files have both Copyright AND (currently missing from some) a statement of copying permission, saying that the program is distributed under the terms of the GNU General Public License.